V1369 Cen

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Re: V1369 Cen

Postby Malcolm Locke » Tue Jan 21, 2014 10:01 am

Attached is a plot of the evolution of the absorption systems in the Hydrogen beta line. Velocity zero point has been set to 4861 Angstrom for all plots. Data is taken from the ARAS database from spectra provided by Ken Harrison and Terry Bohlsen.

There is a clear progression upwards in velocity over the 25 days covered, with the main absorption feature centre moving from around -1500 km/s to around -2100 km/s, and the secondary feature moving from around -750 km/s to around -1200 km/s. The main feature now extends to a maximum negative velocity of around -2500 km/s.

The absorption visible on the red wing of the Hb line in the latter spectra is the high velocity feature from FeII 4923 and has a similar velocity to the Hb feature.

Thanks as always to Ken and Terry for the data.

hb_velocities_20131224-20140118.png
hb_velocities_20131224-20140118.png (17.99 KiB) Viewed 7924 times
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Re: V1369 Cen

Postby Terry Bohlsen » Wed Jan 22, 2014 10:29 am

Great graph Malcolm.
I will have to learn to use IRAF.

Terry
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Re: V1369 Cen

Postby Robin Leadbeater » Wed Jan 22, 2014 10:39 am

Do we know what is happening here? It seems odd that the material should be accelerating, or are we seeing deeper into the material as it thins and cools to see layers moving at higher velocity ?

Cheers
Robin
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Re: V1369 Cen

Postby Malcolm Locke » Thu Jan 23, 2014 8:42 am

Robin,

This is one part of the evolution I definitely don't understand!

It's well documented that this change in the observed velocities is a normal part of the evolution.

The idea that this is due to seeing deeper into the expanding ejecta doesn't make sense in my interpretation of what I have read. If there is a velocity gradient in the ejecta (Steve's notes from nova del indicate this) then the outer layers of the ejecta will be moving faster, and the inner layers moving slower, so as we saw deeper we would see a decrease in velocity, not an increase.

Steve's paper 'Spectroscopy of Novae - A Users Manual' has some figures (fig 4 & 5) that show a similar evolution. There it is attributed to a 'recombination front' - I'm afraid I can't pretend to comprehend the accompanying text though, or be sure that what is shown in the plot above is the same phenomenon.

If anyone has a dummies explanation I'm all ears :)
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Re: V1369 Cen

Postby Tasso Napoleao » Thu Jan 23, 2014 3:29 pm

Dear friends,

140121_novacentauri_600s.png
140121_novacentauri_600s.png (19.86 KiB) Viewed 7898 times


The evolution of the expansion velocity based on our H-Alpha profiles follows the same trend already indicated by Malcolm and based on H-beta. Since January 03 and up to Jan 21, we have estimated an increase in expansion speed from about -1800 km/s to some -2100 km/s. We would also appreciate to hear from our more experienced fellows about what is actually going on here…

Cheers,
Tasso.
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Re: V1369 Cen

Postby Malcolm Locke » Fri Jan 24, 2014 10:16 am

Spectrum from 22nd Jan attached. My first effort with a slit spectrograph, so a little bit shabby I'm afraid. Still there is some useful data hiding in there.

16" Meade RCX400, L200 600 l/mm, Canon 550d camera, unguided. Focus is a little soft in the red end so the Ha line has lost some of its intensity.

The velocity profile is from Hb.

v1369_cen_20140122.png
v1369_cen_20140122.png (15.04 KiB) Viewed 7891 times


V1369_cen_Hb_velocity_20140122.png
V1369_cen_Hb_velocity_20140122.png (9.62 KiB) Viewed 7891 times
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Re: V1369 Cen

Postby Francois Teyssier » Sat Jan 25, 2014 7:41 am

Congratulations Malcom
This is excellent as a fisrt sepctrum
Just a little bit noisy in the continuum (how many images did you stack to get this result ?
Of course, I"m wating for the fit file for the data base
Bonne continuation
François
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Re: V1369 Cen

Postby Malcolm Locke » Sat Jan 25, 2014 9:36 am

I'm working on it François, should be with you shortly ;)

7 x 15 second exposures. Lack of guiding is a severe issue, doesn't take long for the star to drift off the slit. I will work on this.
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Re: V1369 Cen

Postby Jon » Mon Jan 27, 2014 1:13 am

Malc, Robin, Tasso,

There seem to be two explanations - or two possible causes - as to why we see an increasingly blue-shifted set of absorption lines in this phase of nova evolution. One (the simpler to understand, at least by me!) is advocated by Bob Williams (see Williams and Mason, 2009, Nova ejecta as colliding shells, http://arxiv.org/ftp/arxiv/papers/0908/0908.3810.pdf):

"[We see] multiple absorption systems having different radial velocities in the spectra of a large majority of novae and interpreted the observations in terms of discrete clouds of gas that are progressively accelerated outward. The innermost ejecta, having the higher velocities, correspond to the surface layers of the white dwarf (WD) that have undergone nuclear reactions. The outermost ejecta are circumbinary gas having lower velocities, and their presence at maximum light surrounding the primary WD ejecta strongly suggests that the outer circumbinary transient heavy element absorbing (THEA) gas has existed before the TNR." (Williams and Mason, 2009 p.1)

Basically he argues that previous outflow from both the WD and in particular the secondary star creates a circumbinary gas envelope moving outwards from the pair at relatively low velocities. This is successively energised (ionised, heated, and collided with) by the initial nova outburst hence is the first set of absorption (and emission) lines we see. Later, as the fireball become less optically thick, we see faster moving lines from the actual ejecta themselves. The attached diagram is from p.7 of that article.

williams diagram.jpg


One of the problems with this theory seems to be that the energy required to push this much material out into the circumbinary envelope is substantial, and we see little evidence of this sort of emission from the system prior to the nova outburst. But it seems true to say that the recent literature seems to be dominated by discussion of the evidence for and against some sort of steady-flow stellar wind (as opposed to a ballistic expansion) as a contributor to novae spectral evolution.

Steve Shore posits another explanation for Discrete (blue-shifted) Absorption Components based on his study of the T Pyx outburst in 2011 (Shore et al, 2011 http://arxiv.org/pdf/1108.3505.pdf). Shore argues that the velocity structure observed in the ejecta is "frozen" in the structure of the initial explosion - that is, it's all there from the start. However, as Robin suggested, different absorption components become visible as specific transitions, particularly in the UV, become less optically thick, while a recombination front moves outward through the ejecta. However, the point is, we're not "seeing further in", as much as, as things thin, more differentiated structure is able to *get out* - gradually picking up structure in the outer/faster envelope.

I won't begin to suggest that I understand all the physics in this latter case. But a few things it helped me to remember when thinking this through. One is that optical thickness varies by wavelength, so the velocity structure we observe down near the UV will not be the same as that up in the IR. The other is that we are seeing the cumulative effect of superimposed shells of absorption. So, let's assume for simplicity that the inner (slower - let's say -400 km/s) ejecta start off absorbing 100% of the photons between 4830 and 4861 - we'd therefore see no faster absorption line sat all, just a band at -400 km/s. As these ejecta thin, more photons will get through to be absorbed by the outer (faster) ejecta, thus we'd begin to gradually see more, thinner, and faster lines as the ejecta thin. And this is exactly what we see.

Finally, the recombination front. This is particularly relevant to hydrogen, but that's what we're looking at. Obviously, ionised hydrogen can't create absorption lines. So as the recombination front (which I'm assuming is an expanding circumference in the ejecta which is the point at which energy levels have dropped enough to allow significant recombination of the hydrogen plamsa to occur) moves outward from the WD, it begins to reach parts of the ejecta that are further out (and thus faster-moving). As the hydrogen in successively further and faster regions recombines, it will create emission lines; but it will also therefore provide a supply of neutral hydrogen to create absorption lines.

I must say I found this notion of the "recombination front" very helpful in understanding why the absorption lines velocities appear "inside out" and get faster with time.h
Cheers

Jonathan
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Re: V1369 Cen

Postby Terry Bohlsen » Tue Jan 28, 2014 10:03 am

Spectra from last night. The P Cyg profiles are mostly gone. It has brightened in Ha in absolute flux but has reduced otherwise.
Image

Can someone tell me how I make gnuplot change the Y axis so that it displays scientific notation rather than lots of zeros.
ie 1 E-10 rather than 0.0000000001
I'm using the std2.gnu file in ISIS.
Terry
Image
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